1. Field of the Invention
The present invention relates to a printing apparatus provided with a paper supply unit for supplying paper sheets fed one by one in a predetermined direction by means of a supply roller from a paper accommodating unit such as a hopper, and more particularly to a printing apparatus capable of removing back-tension generated in a paper sheet before printing characters and the like on the paper sheet by a print head while feeding the paper by rotation of a step motor, to maintain a constant paper feeding accuracy and perform printing with high quality.
2. Description of Related Art
Heretofore, there have been proposed various printing apparatuses such as a printer and the like for performing printing of characters and the like by using a print head while feeding paper sheets one by one. In such the printing apparatus, a plurality of paper sheets are generally accommodated in a stack state in a hopper. At the time of printing using the print head, a pick-up roller disposed above the hopper is in contact under pressure to the uppermost positioned paper sheet and feeds only that paper sheet in cooperation with a supply roller toward a printing position where the print head prints characters and the like on the paper sheet.
For a rotating mechanism of the paper supply roller, commonly, a step motor and a gear mechanism which transmits rotational motion of the step motor to the paper supply roller. During printing characters and the like, the printing apparatus alternately conducts an operation of rotating the step motor by predetermined steps in a paper feeding direction to thereby make a rotation of the supply roller by a predetermined paper feeding amount and another operation of performing one-line printing by the print head. At this time, a so-called back-tension is generated in the paper sheet due to the contact under pressure with a collar supported coaxially with the pick-up roller, the backlash generated in the gear mechanism, the bending of roller shafts of rollers of the paper feeding mechanism including a roller shaft of the paper supply roller, the mechanical backlash generated between components. In the above conventional printing apparatus, therefore, a paper feeding is conducted by the paper supply roller rotated by the step motor so as to produce the paper pulling power larger than the back-tension.
As mentioned above, the backlash generated in the gear mechanism, the bending of the roller shafts of rollers of the paper feeding mechanism, and the mechanical backlash generated between components inevitably exist in the printing apparatus; however, the back-tension to be generated in the paper sheet as above has characteristics of being determined based on a balance with the above-mentioned backlash and others. Accordingly, as long as the supply roller is rotated in one direction (the paper feeding direction), the backlash and others are constant. As a result, a constant paper feeding accuracy can be maintained when an operation of rotating the step motor by predetermined steps in the paper feeding direction to rotate the paper supply roller by a predetermined paper feeding amount and another operation of performing one-line printing using the print head on the paper are repeated alternately. This can prevent the occurrence of white lines in printed characters owing to missing print dots.
However, as printing is continued on the paper sheet supplied via the supply roller, the paper sheet is sequentially fed and then is released from the collar supported coaxially with the pick-up roller, when the back-tension having occurred in the paper sheet until then is removed. The supply roller and the print head are positioned apart at a predetermined distance in view of their mechanisms, whereby the print head can conduct printing on the paper sheet even after the back end of the paper sheet is released from the collar. Removal of the back-tension generated in the paper sheet causes the unbalance between the back-tension and the backlash occurring in the gear mechanism and others, so that the force caused by the backlash and the others acts on the paper sheet in the paper feeding direction, thus deteriorating the paper feeding accuracy. Accordingly, the paper sheet is fed more than necessity, causing white lines in characters printed on the paper sheet and therefore deteriorating printing quality.